The proposed work will develop the hypothesis that the concentration of acetylcholine (ACh) at postjunctional effector sites in the atrium regulates the expression of beta-adrenergic -cyclic AMP mediated functions. Isoproterenol stimulated cyclic AMP accumulation in isolated murine atria is markedly inhibited by K ion -induced depolarization, cholinesterase inhibition or carbachol. All of these mainpulations block cyclic AMP accumulation through a muscarinic (atropine-sensitive) mechanism. I propose to demonstrate that elevated K ion releases ACh from prelabelled (with 3H-choline) neurotransmitter stores in isolated atria, and that subsequent inhibition of cyclic AMP accumulation is proportional to the available released ACh. By quantitation of the effect of released or exogenous ACh, I will determine whether there is noncompetitive antagonism of catecholamine stimulated cyclic AMP accumulation by muscarinic receptor activation. Isoproterenol induced activation of protein kinase and phosphorylase will also be known to be inhibited in proportion to the concentration of choline ester. I will examine local presynaptic mechanisms by which drugs and homones might modulate K ion -induced ACh release, as potential physiological regulators of the expression of cyclic AMP dependent beta-adrenergic functions. Developmental and hormonal changes in the density or sensitivity of muscarinic receptors will be utilized to examine postsynaptic cholinergic receptor mechanisms regulating adrenergic responses. Other studies will focus on changes in cyclic GMP, phosphodiesterase, and GTP'ase activity as possible bases for muscarinic antagonism of cyclic AMP accumulation. The possibility that elevated K ion release ACh, and thereby modulates beta-adrenergic responses, in papillary muscle and in prefused hearts will also be examined. The eventual goals of this work will be to demonstrate that the dominance of parasympathetic influences in the atrium can be explained by cholinergic inhibition of cyclic AMP accumulation, and that intracardiac cholinergic neurons are critical effector sites for drug and hormone actions on cardiac muscle.